When products including expanded plastic material are being manufactured it is essential, considering economy as well as quality, that the plastic material has a correct volumetric weight. That is, that the material has been allowed to expand to a predetermined degree because a volumetric weight which is too high means higher production costs, and a volumetric weight which is too low, results in a bad product. Therefore, it is necessary to follow up carefully the volumetric weight of expanded plastic material which is used in product manufacture.
The manufacture of insulating slabs of expanded polystyrene is one example of a product for which it is very important to follow carefully the volumetric weight of the plastic material included therein. Said product is standardized and it is required, for example, for an insulating slab which should have nominally a volumetric weight of 15 kg/m.sup.3 that the volumetric weight is within the range from 14.5 to 15.5 kg/m.sup.3. The basic material is granulated polystyrene having a volumetric weight of 600 to 700 kg/m.sup.3, and this material is pre-expanded in one or more stages by a continuous or discontinuous process to a volumetric weight of 15 to 20 kg/m.sup.3 before it is introduced into the machine for manufacturing the slabs of expanded polystyrene. Further expansion of the pre-expanded material takes place in order to sinter together the material so as to form homogeneous blocks or slabs. When no accurate control of the volumetric weight is applied one is inclined to maintain said weight in the center of the volumetric weight range established by the prescribed standard so as to be on the safe side. However, when an accurate control is applied the volumetric weight can be maintained closer to the lower limit. This implies a reduced consumption of the basic material and thus a reduction of costs, which is not unimportant if the manufacture of large quantities is concerned. At the same time there will be obtained, without doubt, an acceptable product, thus less scrapping and fewer complaints.
By the method according to the invention it is well suited to be integrated with existing manufacture processes, said accurate control of the volumetric weight of the expanded material is made possible.
For practicing the method the invention also relates to an apparatus.